Corundum is a simple mineral, also called adamantine spar. Its specific gravity varies from 3.975 to 4.161. It contains about 90 per cent, of alumina, a little silica, lime, magnesia, and water. It is insoluble in acid, infusible by the blowpipe flame, but fuses gradually when heated with flux. It is generally found in ill-defined crystals, or acute and obtuse hexahedral pyramids, and is of a pale grey or greenish colour, also blue, red, and brown. It ranks in hardness next to the diamond, the sapphires being the blue variety and the oriental ruby being the red. It is found in India and in sands of rivers and alluvial matter in Ceylon. Common corundum is found iu granitic rock in India, Mont St. Gothard, and Piedmont. The granular variety, containing peroxide of iron, is the emery of commerce, found in the Isle of Naxos in rolled masses at the foot of primitive mountains.